1) Field of the Invention
The invention relates to a process for preparing homo- and copolymers of vinyl chloride by batch polymerization in aqueous emulsion in the presence of an emulsifier combination made from at least two emulsifiers selected from the class consisting of the anionic and nonionic emulsifiers, metered in during the polymerization.
2) Background Art
Pastable homo- and copolymers of vinyl chloride, which are suitable for producing plastisols, are prepared by emulsion polymerization or microsuspension polymerization in the presence of one or more emulsifiers. The type and amount of the surfactants used in emulsion polymerization affects not only the paste viscosity significantly, but also the properties of the final polymer product, for example transparency, antistatic properties, foamability and moisture absorption. In many cases, the properties required in the final product cannot be achieved using just one emulsifier, but only with emulsifier mixtures.
To this end, the initial charge in single-step polymerization processes comprises emulsifier mixtures. In surfactant-feed polymerizations, they are metered in during the polymerization procedure. One or more other emulsifiers may also be added afterward to the completely polymerized latex before spray drying.
A single-step emulsion polymerization process in which the initial charge comprises an emulsifier mixture is described, for example, in DE-A 2801575 (U.S. Pat. No. 4,071,675). A disadvantage here is that single-step emulsion polymerization of vinyl chloride generally gives very small polymer particles. The resultant latices have low latex stability and paste PVC prepared from them is highly viscous.
To improve latex stability and paste viscosity, the prior art recommends that the emulsifiers be metered in, and a wide variety of different versions has been described. It is known from DE-A 3343766 and EP-A 90142 (U.S. Pat. No. 4,631,324) that pastable vinyl chloride polymers can be prepared using an initial charge which comprises part of the emulsifier mixture and metering in the remainder of the mixture during the course of the polymerization. DE-A 2633833 and DE-A 3049008 describe processes in which emulsifier mixtures are metered in during the polymerization as a function of the polymerization conversion or of the surface tension of the polymerization mix. The polymerization of paste PVC in the presence of seed latex and of an emulsifier mixture is described, for example, in DE-A 2542283 (U.S. Pat. No. 4,046,730). The initial charge here comprises the seed latex and one portion of the emulsifier mixture or one component of the emulsifier mixture, and the remainder of the mixture or the other components of the mixture are metered in during the polymerization. The disadvantage of metering in mixed emulsifiers in the manner mentioned is that at least one component of the surfactant mixture is generally used for its influence on final product properties. Its use during the polymerization results in an adverse effect on rheology, i.e. higher paste viscosities generally have to be accepted.
When emulsifiers are metered in afterward into the completely polymerized latex, that is to say additional surfactant is metered in before or during the spray drying, the desired rheological properties of the polymers remain substantially unaffected, but this additional feed results in a very high total surfactant content. However, high surfactant concentrations in the polymer cause undesirable side effects, such as greater moisture absorption (unacceptable in particular for outdoor applications), color changes, viscosity variations, impairment of transparency and exudation phenomena linked with surface gloss, odor, fogging and tack.
DE-A 1745485 (U.S. Pat. No. 3,546,154) describes a process for emulsion polymerization of vinyl chloride in which, in order to obtain polymer dispersions with very large primary particles, the surfactants added at the start of or during the polymerization are exclusively poorly emulsifying, short-chain surfactants with a chain length from 5 to 9 carbon atoms, and the addition of long-chain surfactants with good emulsifying action does not take place until a second phase of polymerization or after the polymerization has been completed. A disadvantage is that the poorly emulsifying short-chain surfactants ensure no more than a weak stabilization of the polymerizing latex, and therefore coagulation can easily occur if there is strong agitation, and on the other hand if the agitation is weaker separation of the phases is likely. In addition, if the polymerization is carried out to relatively high conversions with exclusively poorly emulsifying surfactants, relatively large amounts of emulsifier have to be added, and these are still further increased when the latex has been stabilized with a strong surfactant.
It was therefore an object of the invention to develop a process which serves for the preparation of homo- and copolymers of vinyl chloride by batch polymerization in aqueous emulsion and which, even if the emulsifier combinations used comprise surfactants used to achieve desirable properties in the final product, minimizes the paste viscosity obtainable with the polymerization product and at the same time gives a low total surfactant concentration.